I'm guessing that the problem in doing that is that the OP stated "because I have limited mobility and depend a lot on delivery..." That limited mobility may be a problem in getting to the Subway location.
The OP stated that the phone number given by Doordash for the store was disconnected. The OP may want to try to find a good number for the store with other searches since the Doordash web site number might not have been correct, but assuming it was then calling won't do any good. A letter is worth trying, if the OP can find an address to send it. If the Doordash driver couldn't find the place, then that too may be a dead end.
That's what I'd do, and do it promptly. Hopefully the purchase was by credit card rather than debit card, since credit cards offer more protection in this situation than debit cards do. Sorting it out with a debit card can be much more of a hassle and more time consuming. Federal law provides more protection for credit card disputes than for debit cards.
In at least most states, if you win your small claims case you get the filing fee and service of process fees added to your judgment. The problem though is that it is still up to you to collect that judgment from the franchisee, and that will be difficult if that location does not exist anymore. You might also be able to successfully sue Doctor's Associates, which is the actual name of the company that runs the Subway business because you placed the order through the Subway website. You'd want to read the terms and conditions of the Subway ordering web site to see if there are limits on your ability to do that.
I suggest you try going through your bank first. Explain the situation and the issue with the two orders. If you provide a clear explanation it should not matter which of the two orders actually went through. Then once you have a response from the bank you can decide what to do next.